Unfortunately, you can't just use the earphone/headphone output as a discriminator tap; the audio is way too filtered for it to be of any use by the time you pipe it into a line-in or mic input on a computer for DSD+ to make use of it, that's one reason the technical term for a discriminator tap would be a baseband audio tap because that's the raw unfiltered signal a program like Unitrunker or DSD+ and even the original DSD requires to function correctly.
The only actual option would be to add a discriminator tap on the 396XT and that means opening it up and doing a bit of tinkering (either direct soldering or perhaps using some very tiny alligator clips) - now, since I don't own a 396XT personally everything that I just said is based on the research I just did for a few minutes about the 396XT and a discriminator tap so if I'm mistaken here someone will point that out and I'll just chalk it up to my own ignorance and a failure to find the relevant info.
But I'm pretty sure the only scanner on the market (technically it's not since GRE is out of business) that can provide an actual proper discriminator output through the earphone/headphone jack is the GRE PSR-800 - would have been awesome for Uniden to add that specific feature to the new 436 and 536 scanners just becoming available right about now but afaik they didn't, sadly.
I myself have
never ever had success with any scanner of any kind providing a usable signal off the earphone/headphone jack to mimic what an actual discriminator tap provides.
But I could be wrong, I'm just sayin'...
EDIT:
Ok, so I was correct about the earphone/headphone jack, HOWEVER... the 396XT can pipe a signal through the USB connection that Unitrunker can make use of, even so you're still basically stuck in the sense that it's not a true baseband audio signal that DSD+ requires - it's a serial data stream and not audio, and there's no proper way to convert it from the USB stream to something DSD+ can make use of so again, that's that.